In known manner, an electro-mechanical filter is composed of a mechanical filter formed by a series of resonators interconnected by couplers, together with an electro-mechanical inlet transducer and an electro-mechanical outlet transducer which are both connected to the mechanical filter by couplers. The electro-mechanical transducers transform electric currents into mechanical vibrations or perform the inverse transformation. In a mechanical filter the resonators and the couplers are metal bars, are generally cylindrical, and have defined length and section or diameter, with the couplers being of a smaller diameter than the resonators. More precisely, in a mechanical filter using longitudinally vibrating filters and couplers which likewise vibrate longitudinally, (which is the type of mechanical filter used in the present invention), the resonators in general have a length corresponding substantially to a half wavelength of the chosen resonant frequency and the couplers in general have a length corresponding to a quarter wavelength of the resonant frequency. The diameters of the resonators and of the couplers are likewise precisely defined in order to obtain desired coupling coefficients between the resonators, the coupling coefficient between two cylindrical resonators connected by a cylindrical coupler being proportional to Z.sub.c /.sqroot.Z.sub.r1 .multidot.Z.sub.r2 and consequently to D.sub.c.sup.2 /(D.sub.r1 .multidot.D.sub.r2), where Z.sub.c, Z.sub.r1 and Z.sub.r2 are the impedances of the coupler and the two resonators in question, and where D.sub.c, D.sub.r1 and D.sub.r2 are their diameters whose squares are proportional to the respective impedences. The different desired coupling coefficients can thus be obtained by using resonators of the same diameter and couplers of different diameters, or by using resonators of different diameters and couplers of the same diameter.
Realizations of electro-mechanical frequency pass-band filters can thus be arrived at in which the central frequency of the pass-band is defined by the resonant frequency of the resonators.
Frequency pass-band electro-mechanical filters thus realised, while satisfactory in the pass band, nonetheless presents several drawbacks due in particular to the existence of parasitic modes of vibration which may occupy the pass-band or the attenuated bands of the filter. These modes are essentially bending modes.
The parasitic modes of bending vibration in the resonators and the couplers are a function of the diameters of the resonators and the couplers. It is possible to shift the frequency position of the parasitic modes and thereby move them out of the pass-band of the filter by changing the diameters of the resonators and the couplers. Thus in practice the diameters of the resonators and the filters are chosen in such a way as to satisfy essentially two requirements: firstly, to obtain the desired coupling coefficients between resonators; and secondly, to put the parasitic bending mode vibrations into frequency zones in which they will be of no or little nuisance. In the corresponding filter realizations, the parasitic modes are then positioned outside the pass-band of the filter, however, since the level of these parasitic modes remains unchanged, the unwanted effects of the unattenuated parasitic modes are not always completely and certainly avoided.
The present invention makes it possible to reduce the side effects of the parasitic oscillations, even in cases where it is not certain that they are totally eliminated.